Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K. R. Osborn and W. A. Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
Orientation is accomplished by heating a polymer to a temperature at or above its glass transition temperature Tg, but below its crystalline melting point (Tm), and then stretching the film at a desirable rate. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. Biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. Biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces and tearing; leading to their greater utility in most packaging applications.
Oriented cyclic olefin copolymer films have a number of unique and useful properties. Processing of films including cyclic olefin copolymer on conventional industrial extrusion plants to make films; however, often produces a film having an undesirably high number of defects, such as gelling or fish eyes. Such defects can be reduced by the addition of a polyethylene to the cyclic olefin copolymer. But while including polyethylene reduces the number of defects, the resulting film has an undesirable level of haze. The presence of defects and/or haze limits the usefulness of the resulting films. There is therefore a need for cyclic olefin copolymer films that retain the particular properties imparted by the cyclic olefin copolymer yet have a reduced number of defects as well as low haze.